Chemistry Reference
In-Depth Information
TABLE4.3.UpperandLowerLimitsoftheCoveringand
ThicknessforThreeTypicalMatrices
Matrices
Organic Tissues
Mineral Powders
Metalline Covers
Constituents
1
H...
8
O
8
O...
20
Ca
24
Cr . . .
30
Zn
Covering:
m
max
[
μ
g/cm
2
]
250
140
8
m
min
[
μ
g/cm
2
]
5
5
5
1 0
10
10
Thickness:
d
max
[
μ
m]
12
0.7
0.01
d
min
[
μ
m]
0.015
0.015
0.015
Source
: From Ref. [4], reproduced with permission. Copyright1996 by John Wiley and Sons.
The density of the carrier is to be given in g/cm
3
so that the thickness
d
min
is
calculated in nanometers. This quantity is independent of the excitation mode,
the analyte, and the matrix. For quartz-glass carriers,
d
min
is about 15 nm.
The decisive values for lower and upper limits of the covering and thickness
of a specimen are summarized in Table 4.3 for the three typical matrices
mentioned. The lower limits are generally determined by the detection power
and the height of the first antinode. The upper limits are set by the capability of
the detector, that is, by its maximum count rate. Matrix absorption leads to the
same order of magnitude. Its influence, however, may be reduced if internal
standardization is carried out. The thickness of a specimen can even exceed
10- or 100-fold of the tabulated value of
d
max
if the covering
m
max
is kept
constant. This can be achieved by an incomplete and nonuniform covering.
Even slighter restrictions may be necessary if the quantification is performed
by standard additions. In this case, analyte and internal standard are the same
and the limitation by matrix absorption, expressed in Equation 4.16, becomes
irrelevant. But also here, a homogeneous distribution of the analyte and
standard is desirable in order to get accurate results.
4.4.3.2ResiduesofMicroliterDroplets
The deposition of microliter droplets from diluted standard solutions on flat
substrates like quartz glass or silicon leads to ring-shaped residues rather than
thin homogeneous layers as shown in Section 4.1.3.1. After Hellin
etal
.the
dried residues can be represented by wide hollow cylinders [57,58]. The outer
diameter of a 50
μ
l droplet is typically 1.5 mm, the width of the wall can be
50
μ
m and its height may be 1
μ
m while the central bottom is only 0.15
μ
m
thin. The density of the metal-salt residues is about 2.5 g/cm
3
with a large
variance. The linearity of calibration curves was experimentally proved for
samples of Ni and Ge salts—excited by W-L
β
and Mo-K
α
radiation at 70% of
the critical angle of total reflection. The linearity was also checked for hollow
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